linux-kernel - Re: [PATCH] signal: Avoid corrupting si_pid and si_uid in do_notify

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Date:   Tue, 21 Apr 2020 13:26:06 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linux Containers <containers@...ts.linux-foundation.org>,
        Christof Meerwald <cmeerw@...erw.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] signal: Avoid corrupting si_pid and si_uid in
 do_notify_parent

On Tue, Apr 21, 2020 at 01:11:40PM +0200, Oleg Nesterov wrote:
> Sorry Christian, I don't understand...

In my original mail, it was really just a clarification question. I
said the patch is correct from looking at the codepaths. :) I was just
trying to see whether there was a potential corner-case we're missing
where "force" could be _validly_ true.

> 
> > because child subreapers can't come from outside the pid namespace. If
> > they could you could create a scenario where the signal is generated
> > from a sibling pid namespace in which case it would be correctly set to
> > true.
> 
> not sure I understand, but probably the answer is "yes"...

(This is really purely academic now since it isn't possible, but for
pure amusement assume that a child subreaper could cross namespace
boundaries (which they don't). A marks itself as a subreaper and creates
a new process B in a new pid namespace <pidnsB>, process B setnses into
<pidnsC> which is a sibling pid namespace, B clones a new proces in
<pidnsC> which is now a full member of <pidnsC>, B dies and C is
reparented to A, B exits and then you'd be getting a sigchld from a pid
in a pid namespace in which you have no pid nr.)